1. Field of the Invention
The invention relates to a process for drawing glass fibers for optical communication systems. More particularly, it is concerned with methods for controlling the optical properties and diameter of the optical fiber.
2. Description of the Prior Art
The fabrication of glass fibers with precise optical properties and uniform diameter has become important in recent years because of their potential use in optical communication systems. A variety of procedures have been devised for obtaining glass fibers with these characteristics, including the use of lasers as sources of heat. See, for example, U.S. Pat. No. 3,865,584, issued Feb. 11, 1975 to R. E. Jaeger et al. A particular advantage of using a laser as a source of heat is the ability to direct and focus the laser beam over a limited area using ordinary geometric optics.
Processes for fabricating glass fibers using laser radiation may be divided into two categories. In one category, the preform (glass structure from which the fiber is drawn) is rotated to achieve more cylindrically symmetric temperature distribution. In the second category, optical processing of the laser beam is used to obtain a more cylindrically symmetric temperature distribution. This may be done, for example, by rotation of the laser beam around the preform. Another method depends on the use of a particular laser mode (say a donut mode) for heating the preform. By proper use of lenses and other optical devices, the laser radiation can be uniformly distributed about the preform. The invention is concerned with the second category of processes.
Radial rotation of the laser beam may be achieved by movement of the laser beam and translation of this movement into radial rotation by appropriate use of geometrical optical devices. In the above-described reference, a specific system is described which achieves radial rotation of the laser beam about the glass preform. A description of this optical system may be helpful in understanding the prior art.
The source of heat can be an infrared laser with radiation frequency in the spectral range where the glass is strongly absorbent. Typical is the use of the CO.sub.2 laser with output at 10.6 .mu.m. The output of the laser is directed through a rotating lens and onto a planar reflector.
Although the planar reflector may be mounted in a variety of ways, it is preferred for symmetry reasons to have the planar reflector mounted to 40.degree. from both the laser beam and the axis formed from the pulled fiber and glass preform. The lens is rotated eccentrically at high speeds about the axis of the laser beam. This produces a more or less circular motion to the laser beam (when viewed in a plane orthogonal to the laser beam). The rotating laser beam is reflected off the 45.degree. mirror to a conical reflector which directs the laser energy onto the glass preform. The glass preform is fed through the back of the conical reflector and the glass fiber is pulled toward and through a hole in the 45.degree. mirror. By rapid rotation of the lens (typically in excess of 1500 rmp) which results in radial rotation of the laser beam around the glass preform, the heating of the preform is made cylindrically more uniform.
Another method of obtaining cylindrically uniform distribution of laser heat on the preform without beam rotation involves use of a donut mode in the laser and the use of lenses and a conical reflector to direct the laser radiation onto the preform.
Although excellent diameter control can be achieved using a rotating laser beam and the optical system described above, improvements in the short term control of diameter, the long term control of diameter and process stability are highly desirable. Improved diameter control ultimately leads to lower losses and less scatter. Because of the shape of the conical reflector, the laser radiation reaching the glass preform is focused in the form of a line along the axis of the preform and drawn fiber. This concentrates equal radiation on both the massive glass preform and the partially drawn glass fiber causing much higher concentration of radiation on the drawn fiber. Improved diameter control can be achieved by a more suitable distribution of laser radiation on the glass preform and glass fiber being drawn. Also, better matching of the laser radiation to the glass preform and drawn glass fiber should result in a more stable fiber drawing process which is especially advantageous under conditions of glass fiber manufacture.